Increasing development in the Arctic and sub-Arctic has emphasized the need to minimize the adverse effects of seasonal breakup in the ice cover of rivers and lakes. These effects include the flooding caused by ice jams and the impact of ice on structures such as man-made bridges and natural obstacles such as rocks and fallen trees.
Ice jam floods are usually associated with the spring breakup period when seasonally warming temperatures melt the winter snow cover and dramatically increase the flow in rivers and streams. Most of the larger streams have at this time seasonal ice covers ranging from a few inches to more than 20 feet in thickness. Typically the ice cover at the start of breakup is composed of cold, consolidated, strong ice. Much of the winter ice cover is frozen fast to the stream bed and banks. The early breakup water flow is often over this ice surface. As the breakup flow increases and the seasonal ice cover warms and weakens, the ice releases from the bed and banks and begins to float downstream with the current. These floating sheets of ice may be quite large. They may have widths equal to the winter width of the stream or river channel and lengths up to the length of the straight river reach in which they were formed. This length may typically be four or five times the width of the ice flow. The thickness of these sheets of consolidated ice may approach the maximum winter ice thickness.
Frequently, in larger rivers, these large, strong sheets of ice (ice floes) impinge and ground (stop) on a downstream stream channel man-made structures such as bridges, natural features such as a bend in the river or gravel bar. Small ice floes accumulate at the upstream edge of the original ice sheet. If the momentum of the arriving ice floes is sufficiently high, they may be drawn undeneath. If not, they accumulate upstream. The stream flow deepens and the velocity of the water decreases until such time as an equilibrium ice jam thickness develops. This ice jam may or may not completely block the stream. Damage results from injury to man-made structures such as bridges, flooding, and, occassionally, diversion of the ice itself to overbank property. The ice jam will persist until such time as the strength of the ice cakes are not sufficient to overcome the internal stress in the jam imposed by the increase in river stage.
Another source of damage occurs when moving ice floes impact on man-made hydraulic structures such as bridge piers, wharfs, or river training works. The force exerted by ice on these structures is often the dominant force for which Arctic riparian structures must be designed. The destruction of an ice floe by a vertical small hydraulic structure such as a bridge pier occurs in different ways, depending most on the stored kinetic energy of the floe and the ice strength. Upon impact, an ice floe is first locally crushed. If the floe cannot be diverted, the floe will continue to fail either by splitting or, for large floes, by crushing. If an ice floe strikes a structure, the energy transmitted to the structure is made up of the energy of deformation inside the ice floe and the energy required for crushing of the ice. The energy of deformation is negligible compared with the energy of crushing for cases approaching design conditions. A fundamental relationship of physics provides that the acceleration of the ice sheet is proportional to the resultant force on the structure and is applied in the same direction as the force. A second fundamental relationship provides that the force exerted on the structure by indenting and crushing ice is the product of the crushing strength of the ice and the edge area of the crushed ice sheet. From the above two relationships, one may derive the maximum horizontal force which a structure must exert to stop an ice floe. This force is a function of both the size and strength of the approaching ice sheets. There is a minimum mass of ice floe necessary before the maximum crushing force can be developed. Similar relativity exists for ice failure in the tensile and shearing modes. However, the crushing mode is usually the most severe condition. From the above, it can be seen that it is desirable to reduce the size of strength of the ice floes.
The crushing, as well as the shearing and flexural strength of an ice cover, is a function of the crystal structure of the ice which is in turn a function of the temperature of the ice. With increasing temperature, the ice tends to reform its crystal structure towards large vertically oriented crystals. The impurities in the ice tend to concentrate at the crystal boundaries forming wet surfaces which are easily split. As a result, the crushing strength of ice is close to zero at the melting point but rises rapidly in proportion to about eight tenths power of the decrease in temperature below freezing.
Ice may be warmed and thus weakened as a result of one or all of the following physical processes, each of which may be significant during the spring breakup period. Heat may be transferred to an ice mass as a result of condensation of moisture in the air on the ice surface, by solar radiation, and from the adjacent air or water mass. This conductive transfer may be assisted by convention transfer. Heat may be gained because of friction from adjacent flowing water. Other modes of heat transfer exist. The heat transferred to the ice first warms the ice surface and then penetrates into the ice in accordance with the normal laws for heat transfer by conduction. Once the ice reaches the freezing point, any additional heat transferred is absorbed in providing the latent heat of fusion requirement, and the ice converts to water.
In the past, several schemes for reducing the strength and mass of the seasonal ice cover prior to breakup have been employed. These techniques have been employed both to reduce the size and strength of the ice floes. One technique is to apply a thin coating of a dark substance such as coal dust, fly ash or silt to darken the ice surface thereby increasing the amount of solar energy absorbed by the ice. This method depends largely on the ice surface remaining snowfree after the application of the dust, etc. as well as upon the weather sequence subsequent to the application.
Another technique employed consists of breaking up the ice cover by means of explosives. This technique has been employed both with explosives placed on or under the ice surface by hand and with explosives droped from airplanes. This method is of limited application because of the impact on fish and other wildlife and because of the risk to man and his property.
A third technique applicable only to large navigable rivers consists of using breaking vessels to break ice jams as they form. This technique is not always useful around bridges and the like.